They were not sure themselves when they start using HTML5 and the other point is, at that time HTML5 wasn't mature enough to be used specially in mobile devices.
Essentially they move to so called native but that's not true, it's still hybrid, approx.70% html/css and30% native (Objective-C), check out the detail answer by Sencha folks at below links.
http://www.sencha.com/blog/the-making-of-fastbook-an-html5-love-story
They have made super fast Fastbook using HTML5 only and it's awesome.
